In many situations where temperature control is required, an analog feedback type temperature control method is generally adopted, for example, NTC (also known as Negative Temperature Coefficient thermistor) feedback heater temperature value is adopted and an analog circuit negative feedback principle is utilized to form a closed loop control system.
However, this method adopts a resistor to set the finally reached stable temperature of a system, as long-term drift exists in the resistor, long-term drift inevitably exists in the final stable temperature of the system, thus, long-term temperature accuracy of the system is hard to be controlled, and large technical defect exists; and this method is generally only used in the situation that the requirement of system temperature control precise is not so high.
Another digital temperature control circuit adopts a precise resistor and a PID (also known as Proportional Integral Derivative) algorithm to dynamically adjust the system temperature in real-time. This method adopts a precise resistance temperature sensor, thus, the long-term drift problem of the temperature sensor can be avoided. However, as this control circuit is of digital type, the heating current in the heater is often switched between full power and zero power, the pulsation is quite great; when the digital temperature control circuit with such a structure is applied into medical equipments, it will bring great interference to the precise signal measurement circuits of the medical equipments (such as a blood gas analyzer).
In addition, as the temperature control system needs to operate PID algorithm in real-time, the load of CPU is great, and the interference problem brought is also more prominent. This way is hard to solve the inherent interference problem. In addition, once the CPU does not work, the heating current possibly keeps at the maximum all the time to bring potential safety hazards. To solve this problem, a series of protection circuits are needed to add, the system is complex and high in cost. Thus, this way has many disadvantages which are hard to be overcome.
To sum up, the traditional temperature control technology has many defects.